libcamera: Add Semaphore class

Add a general-purpose counting semaphore class.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>

src/libcamera/semaphore.cpp

@@ -0,0 +1,103 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+/*
+ * Copyright (C) 2019, Google Inc.
+ *
+ * semaphore.cpp - General-purpose counting semaphore
+ */
+
+#include "semaphore.h"
+#include "thread.h"
+
+/**
+ * \file semaphore.h
+ * \brief General-purpose counting semaphore
+ */
+
+namespace libcamera {
+
+/**
+ * \class Semaphore
+ * \brief General-purpose counting semaphore
+ *
+ * A semaphore is a locking primitive that protects resources. It is created
+ * with an initial number of resources (which may be 0), and offers two
+ * primitives to acquire and release resources. The acquire() method tries to
+ * acquire a number of resources, and blocks if not enough resources are
+ * available until they get released. The release() method releases a number of
+ * resources, waking up any consumer blocked on an acquire() call.
+ */
+
+/**
+ * \brief Construct a semaphore with \a n resources
+ * \param[in] n The resource count
+ */
+Semaphore::Semaphore(unsigned int n)
+	: available_(n)
+{
+}
+
+/**
+ * \brief Retrieve the number of available resources
+ * \return The number of available resources
+ */
+unsigned int Semaphore::available()
+{
+	MutexLocker locker(mutex_);
+	return available_;
+}
+
+/**
+ * \brief Acquire \a n resources
+ * \param[in] n The resource count
+ *
+ * This method attempts to acquire \a n resources. If \a n is higher than the
+ * number of available resources, the call will block until enough resources
+ * become available.
+ */
+void Semaphore::acquire(unsigned int n)
+{
+	MutexLocker locker(mutex_);
+	cv_.wait(locker, [&] { return available_ >= n; });
+	available_ -= n;
+}
+
+/**
+ * \brief Try to acquire \a n resources without blocking
+ * \param[in] n The resource count
+ *
+ * This method attempts to acquire \a n resources. If \a n is higher than the
+ * number of available resources, it returns false immediately without
+ * acquiring any resource. Otherwise it acquires the resources and returns
+ * true.
+ *
+ * \return True if the resources have been acquired, false otherwise
+ */
+bool Semaphore::tryAcquire(unsigned int n)
+{
+	MutexLocker locker(mutex_);
+	if (available_ < n)
+		return false;
+
+	available_ -= n;
+	return true;
+}
+
+/**
+ * \brief Release \a n resources
+ * \param[in] n The resource count
+ *
+ * This method releases \a n resources, increasing the available resource count
+ * by \a n. If the number of available resources becomes large enough for any
+ * consumer blocked on an acquire() call, those consumers get woken up.
+ */
+void Semaphore::release(unsigned int n)
+{
+	{
+		MutexLocker locker(mutex_);
+		available_ += n;
+	}
+
+	cv_.notify_all();
+}
+
+} /* namespace libcamera */